It’s spring. The weather is turning beautiful. The days are getting longer. It really puts you in the mood for a film that’s . . . scary? Yup. While the premiere of a horror film in April may seem out of place, that’s what we have here, and a pretty good one to boot.

Not so much surprising as effective in what it’s trying to do. Challenging as well, with many moving parts that don’t always fit together. But Oculus is more about mood and tempo. Like a good piece of music, it creates layers, starts slowly and builds to a startling crescendo.

Director Mike Flanagan chooses to use parallel stories — one from the past, one in the present — to get his point across. Two young siblings, older sister Kaylie (Karen Gillan) and younger brother Tim (Brenton Thwaites), watch in horror as their mother, Marie (Katee Sackhoff), is chained up and shot to death by their father, Alan (Rory Cochrane). Eventually, the conflict comes to a head when the father himself is shot and killed by Tim. He is subsequently committed to a mental institution and presumed to be guilty of murder.

Or is he? Kaylie is convinced that Tim is not responsible. The real culprit is the nasty spirit inside the antique mirror in their father’s office. To prove her point, Kaylie spends the next 10 years keeping tabs on the mirror and waiting for her brother to be released. Her plan is to gather with her brother in their old house and try to kill this thing off once and for all. How? Well, this is the part that gets a bit dicey. Something involving lights, cameras, timers and one hell of a big anchor.

There’s nothing here that you haven’t seen in other ghost stories. What works best is the way that Flanagan pieces the story together and keeps you guessing. Owing, I suppose, to his extensive work as an editor in film and television, he cuts between past and present skillfully and uses an ever-increasing pace between them to draw you in.

He also creates just enough confusion to leave you wondering about whether you’re in the past or present, whether it’s the sister or the brother who is mentally unbalanced, and whether this mirror spirit thingy actually exists. Maybe it does. Or maybe, as in any decent horror film worth its screams, maybe it’s just a figment of their imagination.

There are some plot points that are kind of murky, and the tale of the mirror itself gets explained in a breathless rush. I assume the point of the film is not so much the back story about the mirror but its real intent. It doesn’t matter where it came from. All we care about is that it’s there and it’s scary.

Of course, this all might be next week’s joke, except that screenwriters Flanagan and Jeff Howard have managed to do a decent job with the script, and the cast itself is strong enough to pull you in. Scottish cutie Gillan, Brit hunk Thwaites and Battlestar Galactica favorite Sackhoff are convincing enough. Credit also an eerie soundtrack by The Newton Brothers. And something else. There’s an earnest tug here, especially between the younger siblings — sister Annalise Basso and brother Garret Ryan, who are brimming with desperation, terror and the bonding that comes when siblings are under fire from both demented parents and a glittery-eyed ghost.

The ending drives home the real horror: that whatever is in this mirror goes on, until someone else finds it and the terror comes back. And isn’t that always the premise of good horror? Where will evil surface next? Basement? Closet? Attic? Well. Ask yourself. Do you really want to know? My answer? Heck yeah.